Synology has been investigating and working with users affected by a recent ransomware called "SynoLocker." Synology has confirmed the ransomware affects Synology NAS servers running older versions of DiskStation Manager, by exploiting a vulnerability that was fixed in December, 2013, at which time Synology released patched software and notified users to update via various channels.

Affected users may encounter the following symptoms:

When attempting to log in to DSM, a screen appears informing users that data has been encrypted and a fee is required to unlock data.

Abnormally high CPU usage or a running process called "synosync" (which can be checked at Main Menu > Resource Monitor).

DSM 4.3-3810 or earlier; DSM 4.2-3236 or earlier; DSM 4.1-2851 or earlier; DSM 4.0-2257 or earlier is installed, but the system says no updates are available at Control Panel > DSM Update.

For users who have encountered the above symptoms, please shutdown the system immediately to avoid more files from being encrypted and contact our technical support here. However, Synology is unable to decrypt files that have already been encrypted.
For other users who have not encountered the above symptoms, Synology strongly recommend downloading and installing DSM 5.0, or any version below:

DSM 4.3-3827 or later

DSM 4.2-3243 or later

DSM 4.0-2259 or later

DSM 3.x or earlier is not affected

Users can manually download the latest version from our Download Center and install it at Control Panel > DSM Update > Manual DSM Update.
Synology sincerely apologizes for any problems or inconvenience this issue has caused our users. As cybercrime proliferates and increasingly sophisticated malware evolves, Synology continues to devote resources to mitigate threats and is dedicated to providing users with reliable solutions.

Had this on one of my customers Synology NAS's today. The NAS had a backup SR for XenServer. One of our admins followed the instructions and shut it down without realising it had volumes mapped into multiple live servers. He then did a hard reset on the device, leaving us to try and bring the VM backs to a workable state.... Fun day that one....

Lesson learnt. Make sure you update the firmware on these devices regularly and don't port forward the admin panel unless you need to.